Numerous fluoropolymers such as TEFLON, e.g. PTFE, FEP, PFA are now very widely used for coatings, electrical insulation and as structural components due to their superior properties in the areas of hydrophobicity, chemical and thermal stability and electrical insulation. For some applications, fluoropolymers in cellular or foam form are particularly desirable because of their weight, dielectric constant and dissipation factor. In terms of cost savings, the foams are preferred to the non-foams because less fluoropolymer material is used. These properties are often taken advantage of when using fluoropolymer foams in the production of wire products such as coaxial cables and power lines. The applications for these foams may be expanded as the manufacturing processes are improved.
Generally these prior foams are made by processes which include mixing a thermoplastic fluoropolymer, in granular or powder form, with a blowing agent, such as a FREON or diisopropyl hydrazinedicarboxylate, and placing the mixture in a mold. The mold is heated to a very high temperature, for example 240.degree. C. or higher, and subjected to a very high pressure in the order of 1500 psi. After the fluoropolymer reaches a very high temperature it becomes fluid and is moldable and foamable. A variety of methods, including the addition of additives such as nucleation agents, may be employed to facilitate the formation of a particular cell structure.
One example of the prior art in cellular fluoropolymers includes mixtures of polymers or copolymers of perfluorolfins with predetermined amounts of a hydroxyl substituted hydrocarbon. In particular, an aqueous dispersion of a sulfonated lauryl alcohol oxethylate and glycerol, a gas foaming agent, and polyetetrafluoroethylene (PTFE), is whipped until it appears as a stiff foam which is then heat-treated and finally sintered by placing the foam into a 400.degree. oven where the foaming agent is evaporated and the plastic hardened. The final product is an open-cell thermoplastic.
One problem with the prior art foamed fluoropolymers is that the methods of making them often employ blowing agents, nucleating agents, and other additives which may leave undesirable residues, in terms of a particular application, in the foam products in their final form. More importantly, the prior art is restricted to thermoplastic fluoropolymers products which are formed by high temperature sintering processes which are incompatable with the coating or encapsulation of temperature sensitive parts, including some electronics parts.